This invention relates generally to dynamoelectric machines and in particular to a combination starter-protector device therefor, a method of protecting a dynamoelectric machine, a method of operating a dynamoelectric machine, and a circuit.
In the past, various devices and methods have been employed to start a dynamoelectric machine, such as a split phase electric motor for instance, and also to protect the windings thereof in the event of the occurrence of an overload condition which, of course, may have had the deleterious effect of destroying or burning out such windings.
In one such particular past protector device, a positive temperature coefficient resistor (hereinafter referred to as a PTCR) was connected serially with a start or auxiliary winding of a split phase type motor. Upon energization of the motor with the PTCR at a low temperature, sufficient current was passed by the PTCR to energize the start winding thereby to assist in the energization of the motor. Of course, as well known in the art, the PTCR is self-heating upon energization wherein the resistance of the PTCR is increased generally as a function of its increasing temperature. Therefore, at elevated temperatures of the PTCR, it acted to limit passage of current applied therethrough to the start winding to an acceptable minimum value which, in effect, electrically disassociated the start winding from circuit relation with the run or main windings of the motor generally upon the motor attaining its preselected running speed. Of course, one of the disadvantageous or undersirable features of employing a PTCR as a starting resistor is believed to be the time required, after the motor was de-energized or turned-off, for the PTCR to cool sufficiently in order to restart the motor. In some instances, at least one or two minutes may have been necessary to cool the PTCR to a temperature value at which it may pass sufficient current to effect motor restarting.
Another disadvantageous or undesirable feature of at least some of the past protector devices employed with small motors (such as those which may be used to operate refrigerators, freezers or other appliances or the like) is believed to be that they did not have sufficient "off" time to allow the PTCR to cool for restarting the motor when the motor was subjected to a locked rotor condition for instance. A high current draw is encountered under locked rotor conditions, and during the "on" cycle of the protector device, the temperature of the PTCR would, of course, increase. However, when so subjected to such locked rotor condition, the protector device was responsive to the relatively high current draw to revert to its "off" cycle. In the "off" cycle, the protector device was operable generally to electrically isolate the motor windings to prevent short circuiting or burning out thereof. With the windings so isolated from the high current draw, the protector components would cool sufficiently to allow cycling thereof to the "on" cycle which again permitted high current drawn through the windings. In this manner, the motor was cycled continuously by the protector device under locked rotor or high current draw condition which, of course, is believed to be totally undesirable.
In some instances, attempts were made to utilize a PTCR as a starting resistor in various appliances by, whenever possible, placing the protector device on the motor windings and within the housing of a compressor for driving the appliance. Of course, the increased temperature at the protector device within the compressor increased the "off" time to an acceptable value which would permit motor restarting. However, at least one of the disadvantageous or undesirable features of this scheme is believed to be that the cost of installing the protector device within the compressor housing more than offset any savings which may be gleaned by using a PTCR as a starting resistor in the motor circuit rather than a more conventional motor starting relay.